1. Field of the Invention
The present application relates to a method for improving the servo performance of disk drives. More specifically, the present application involves allocating individual spindle motors in a single population of spindle motors to a disk drive design that can be made with different configurations.
2. Description of the Related Art
Disk drives permanently store information in the form of magnetic transitions written onto and read from one or more rigid disks, referred to as xe2x80x9cplatters.xe2x80x9d These transitions are written by a recording head in generally concentric tracks on a magnetic media that is formed on one or more platters. The platters are rotated by a spindle motor. A servo system is provided in the disk drive that includes a plurality of servo sectors on the platters to enable the head to access a particular track. The servo system also enables the head to remain on the track, i.e., to track-follow. Because the track-following performance is an important characteristic of a disk drive servo system, several metrics have been developed to quantify it, including position error signal (xe2x80x9cPESxe2x80x9d).
A variety of factors influence PES performance, including the number of platters mounted on the spindle motor. Disk drive manufacturers typically design a disk drive platform capable of being assembled with one or more platters. By adding one or more platters, the disk drive manufacturer can offer disk drive products having different data storage capacity. Increasing the number of platters in a disk drive generally increases PES because spindle motor runout increases. Non-repeatable runout (xe2x80x9cNRROxe2x80x9d) is one type of runout that appears irregularly. NRRO in the spindle motor is mainly caused by imperfections in the shape of the ball bearings and the supporting parts around them, by their pre-load structure, and by system vibration. Disk drive manufacturers have encouraged their spindle motor suppliers to continually improve the spindle motor NRRO of the population of spindle motors shipped to the disk drive manufacturer to improve PES performance. Spindle motor manufacturers have been asked to ship only disk drives that have NRRO performance that will work in the disk drive configuration with the maximum number of platters. Spindle motor manufacturers also have been asked to adapt to tighter spindle motor NRRO tolerances so that the disk drive designers can achieve higher data track density.
While spindle motor manufacturers historically have adapted to improved NRRO performance requirements, current and future NRRO specifications may not be achievable in a timely and cost effective manner. In any case, the current practice of requiring the spindle motor supplier to ship only a distribution of spindle motors that will work in the worst-case drive configuration results in significant over-design of the spindle motors installed in disk drive configurations having, for example, fewer than the maximum number of platters. It also can result in higher scrap cost, which either reduces the profits earned by the disk drive manufacturer or increases the cost of the disk drive to the customer.
A first aspect of the present invention is a method of manufacturing a population of disk drives. The population of disk drives comprises a first disk drive configuration that has a spindle motor and a second disk drive configuration that has a common spindle motor. A population of spindle motors are manufactured according to a spindle motor design. Each of the spindle motors in the population has a track-following characteristic in a range. The range comprises a first portion and a second portion. A first group of the spindle motors are identified that correspond to the first portion of the range. A second group of spindle motors are identified that correspond to the second portion of the range. The first group of spindle motors are allocated to the first disk drive configuration. The second group of spindle motors are allocated to the second disk drive configuration.
Another aspect of the present invention is an apparatus that allocates a spindle motor from a population of spindle motors according to a track-following characteristic to an appropriate disk drive configuration. The track-following characteristic of the population of spindle motors is in a range. The apparatus comprises a vibration isolating platform that has an electrical connection to which the spindle motor can be electrically connected. The apparatus also has a platter pack, a connector for attaching the platter pack to the spindle motor, and a measurement device that measures the track-following characteristic.